In recent years, the improvement in fuel efficiency of automobiles has been an important subject from the viewpoint of global environmental conservation. In accordance therewith, there has been a movement towards using materials for automobile bodies of high strength and reduced thickness to lighten automobile bodies. However, an increase in strength of a steel sheet reduces ductility, i.e., reduction in forming processability. Therefore, under the present circumstances, the development of materials having both high strength and processability has been desired.
When a high strength steel sheet is formed into a complicated shape such as that of automotive parts, the development of cracks or necking in a bulged portion or a stretch flange portion poses serious problems. Therefore, a high strength steel sheet having both high ductility and stretch flangeability capable of solving the problem of the development of cracks or necking has also been required.
To improve formability of a high strength steel sheet, various multi phase high strength galvanized steel sheets have been developed to date, such as a ferrite martensite dual-phase steel or TRIP steel utilizing transformation induced plasticity of retained austenite.
For example, JP 4-24418, JP 5-72460, JP 5-72461 and JP 5-72462 disclose steel sheets excellent in stretch flange properties by specifying the chemical compositions and area ratios of bainite and martensite or the average diameter of martensite in a three-phase structure of ferrite, bainite, and martensite.
Moreover, JP 6-70246 and JP 6-70247 disclose steel sheets excellent in ductility by specifying the chemical compositions and heat treatment conditions.
The surface of a steel sheet may be galvanized for the purpose of improving the corrosion resistance in actual use. In that case, to secure press properties, spot welding properties, and paint adhesion, a galvannealed steel sheet in which Fe of the steel sheet has been diffused into a plating layer by heat treatment after plating is frequently used. As such a galvanized steel sheet, JP 2007-211280 discloses a high strength galvanized steel sheet and a high strength galvannealed steel sheet excellent in formability and stretch flangeability and a method for manufacturing the same by specifying the chemical compositions, the volume fractions of ferrite and retained austenite, and the plating layer, for example.
However, in JP 4-24418, JP 5-72460, JP 5-72461 and JP 5-72462, the stretch flangeability is excellent, but the ductility is not sufficient. In JP 6-70246 and JP 6-70247, the ductility is excellent, but the stretch flangeability is not taken into consideration. In JP 2007-211280, the ductility is excellent, but the stretch flangeability is not sufficient.
It could thus be helpful to provide a high strength galvanized steel sheet having a TS of 590 MPa or more and excellent processability and a method for manufacturing the same.